1. Field
Apparatuses and methods consistent with exemplary embodiments relate to a technology for evaporative concentration of water, containing hardness-causing substances, to be treated, and more particularly to an apparatus and method for evaporative concentration of water to be treated, in which the water to be treated is evaporatively concentrated by sequential passage through a first evaporator, a hot lime softener and a second evaporator so that hardness-causing substances in the water are effectively removed therefrom by hot lime softening to prevent scale formation, and in which a separate heat source for satisfying the operating temperature of the hot lime softener is not required, thus reducing the operation cost.
2. Description of the Related Art
Zero Liquid Discharge (ZLD) systems are processes in which wastewater is treated so that flowback water (water to be treated) or produced water will be recycled and a small amount of sludge excluding treated water will be discharged to the outside. Recently, companies and academic circles are becoming more and more interested in Zero Liquid Discharge (ZLD) systems.
There are indications of a gradual increase of the supply price of industrial water, a gradual increase of production costs due to an increase of discharge fees by regulations on total quantity of effluent water, and a scheduled enactment of a recycling obligation of more than ⅓ of effluent discharge flow. Also, companies and academic circles value judgments are changing to avoid concerns about environmental pollution problems. Recently, there has been a movement to introduce zero liquid discharge systems not only for specific wastewater but also for all water.
In foreign countries, ZLD systems have been introduced long ago. In Japan, ZLD systems are currently operated in about 100 places. ZLD systems in Japan are installed mainly in high-value-added semiconductor plants. As a large amount of high-quality water is required or the areas of installation of ZLD systems correspond to areas such as national parks in which discharge of specific pollutants is limited, ZLD systems that require high installation and operation costs are introduced and operated.
Particularly, in the case of Canon Inc. (Oita, Japan) that produces copier cartridge products, Oita was incorporated into total emission regulation areas, and thus the neighboring fisheries cooperative association requested the prohibition of wastewater discharge in order to protect fish resources, and for this reason, a ZLD system was introduced in Canon Inc. Meanwhile, UMC Japan, a semiconductor manufacturing plant, is located in a fair park, and thus a ZLD system was introduced therein.
In the case of the USA, it is estimated that ZLD systems are installed and operated in several thousand places. ZLD systems in the USA have been introduced mainly either in area to which strict effluent water quality standards established by each state are applied, or in plants located in areas with poor water supply conditions, such as deserts.
Particularly, La Paloma Plant, a steam power plant located in the middle of the Mojave Desert, Calif., is located in a large agricultural area in which effluent water quality standards are very strict and the supply of water from areas neighboring the Mojave Desert is poor and also the price of water is high. For this reason, a ZLD system was introduced therein, and water recovered from the ZLD system is recycled as boiler feed water to operate the turbine of the power plant.
In the case of Intel Inc., a semiconductor manufacturing plant, a ZLD system was introduced, because emission standards are strict and the Arizona area lacks industrial water.
Such ZLD systems are largely classified into reverse osmosis (RO) ZLD systems that perform separation using osmotic pressure, and thermal ZLD systems that perform separation by thermal evaporative concentration and phase change.
Among them, the thermal ZLD is principally based on the evaporative concentration technology utilized in the 19th century food industry, and intensified environmental regulations leading to increased reuse of water resources have increased demands in thermal ZLD technology applied in various industry fields. The thermal ZLD process using a heat-induced phase change is most effective for non-degradable wastewater.
In recent years, methods for recovering oil sands as alternative resources for conventional oils which are becoming more and more exhausted have been of increasing interest. Particularly, when steam-assisted gravity drainage (SAGD), which is one of these methods, is used, water treatment using the above-described ZLD system is considered a very important process.
This is because water separated in a process of refining recovered oil sands contains contaminants such as silica, and thus causes environmental problems when being discharged without treatment, while it is very efficient to purify water separated from oil sands and recycle the purified water as steam to be injected into oil wells in which oil sands are buried.
Flowback water (or produced water) generated in the process of refining oil sands as described above is generally separated from sludge by an evaporative concentration process. Herein, hardness-causing substances, such as silica, calcium (Ca) and magnesium (Mg), which are present in the water to be treated, can be precipitated during the evaporative concentration process to cause scale on the overall ZLD system including an evaporator. For this reason, a technology for controlling such substances has attracted attention.
FIG. 1 schematically shows a system of the related art which inhibits precipitation of hardness-causing substances when water to be treated, generated in SAGD systems or the like, is concentrated by evaporation.
Referring to FIG. 1, water to be treated, introduced into an evaporator 34 through a water inlet unit 31, is separated into steam and sludge by an evaporative concentration process, and the separated steam and sludge move to the respective discharge units 35 and 36.
Before the water to be treated is introduced into the evaporator 34, the pH of the water is increased by a basic substance such as sodium hydroxide (NaOH), injected from a pH-adjusting substance tank 32. As shown in the graph of FIG. 2, the solubility of silica in water is proportional to pH, and thus precipitation of silica during evaporative concentration of the water in the evaporator can be inhibited.
In another embodiment of the technology of the related art, before the water to be treated is introduced into the evaporator 34, it may be softened by a lime softener 33. In the lime softener 33, hardness-causing substances such as calcium and magnesium are precipitated and separated by reaction with a substance such as lime, whereby softening of the water to be treated is achieved.
Specifically, as shown in the following chemical formula 1, carbonate (HCO3) of calcium reacts with lime (Ca(OH)2 or CaO) to form insoluble calcium carbonate (CaCO3), whereby it can be separated.Ca(HCO3)2+Ca(OH)2→2CaCO3↓2H2O  Chemical Formula 1
In addition, as shown in the following chemical formula 2, calcium bicarbonate (calcium sulfate, calcium chloride, etc.) reacts with soda ash (Na2CO3) to form calcium carbonate, whereby it can be removed.CaSO4+Na2CO3→CaCO3↓+Na2SO4 CaCl2+Na2CO3→CaCO3↓+2NaCl  Chemical Formula 2
Meanwhile, as shown in the following chemical formula 3, magnesium carbonate or bicarbonate (magnesium sulfate, magnesium chloride, etc.) reacts with lime to form insoluble magnesium hydroxide (Mg(OH)2), whereby it can be separated.Mg(HCO3)2+Ca(OH)2→CaCO3↓+MgCO3+2H2OMgCO3+Ca(OH)2→CaCO3↓+Mg(OH)2↓MgCl2+Ca(OH)2→Mg(OH)2↓+CaCl2 MgSO4+Ca(OH)2→Mg(OH)2↓+CaSO4  Chemical Formula 3
In the case of silica, it can either adhere to the surface of magnesium ions which are precipitated by the above-described lime softening process or form calcium-magnesium silicate, whereby it can be separated and removed.
Such lime softening systems can be classified according to operating temperature into three types: a cold lime softener (CLS), a warm lime softener (WLS) and a hot lime softener (HLS). As shown in Table 1 below, the ability to remove hardness-causing substances, which is indicated by the concentration of hardness-causing substances remaining in lime-softened water, increases as it goes from the cold lime softener to the hot lime softener. However, the hot lime softener has a disadvantage in that, because it requires a high operating temperature, it requires a separate heat source.
TABLE 1Type of limeOperating temperatureRemaining concentration (mg/L) ofsoftener(° C.)hardness-causing substanceCLS15 to 60 80 to 110WLS60 to 8530 to 50HLS 90 to 11015 to 25
In the evaporative concentration system of the related art shown in FIG. 1, the cold or warm lime softener is used as the lime softener 33, because there is a limit to pretreating the water, introduced into the lime softener 33, by using an additional heat source. When this cold or warm lime softener is used, the amount of hardness-causing substances removed is smaller than when the hot lime softener is used. Thus, the cold or warm lime softeners has a problem in that it should be used together with the pH-adjusting system as described above so that it can effectively inhibit scale formation caused by precipitation of hardness-causing substances.